1. Field of the Invention
The present invention relates to a communication network for establishing a connection to a user terminal and in particular, but not exclusively, to a user terminal that is capable of roaming amongst a plurality of serving areas in a wireless communication network.
2. Description of the Related Art
It is likely that at some point in the operational life of a communication network, that at least one of the elements is likely to fail resulting in unwanted downtime of the network. Therefore, network designers and telecommunication engineers are constantly striving to find mechanisms that will make a network more resilient to arbitrary failures. Broadly speaking, the elements of a wireless communication network are roughly grouped into being either radio access nodes (RAN) or core nodes (CN) as shown for example in FIG. 1. In practice, the core nodes could for example be MSC's (Mobile Switching Centres).
In practice, the RAN nodes can either take the form of a RNC (Radio Network Controller) in the case of UMTS (Universal Mobile Telecommunication System) network, or a BSC (Base Station Controller) also for GSM, etc.
The known network configuration shown in FIG. 1 has been described in the third generation standard having a specification number 3GPP TS 23.236 and which is entitled “Intra Domain Connection of RAN Nodes to Multiple CN Nodes”.
FIG. 1 shows that the RAN elements are divided into RAN service areas 21, 22, 23, 24, 25, 26, 27, 28 and pool areas 6, 8, 11, 12. The RAN node service areas are also known as LA's (Location Areas), which is the coverage area controlled by the RAN node, i.e. typically an RNC or BSC. On the other hand, each of the pool areas is an area within which a mobile terminal (MT) may roam without needing to change the serving CN node. A pool area is served by one or more CN nodes in parallel. Thus, the configuration of FIG. 1 overcomes the strict hierarchy, which restricts the connection of a RAN node to just one CN node. That is, FIG. 1 shows the RAN node 22, as having a connection to multiple CN nodes, for example MSC1 and MSC4.
Although the 3GPP TS 23.236 standard offers improved redundancy in that a single RAN node can be connected to a secondary CN, this recovers only mobile originated (MO) operations. A mobile originated connection is one in which a call is set up after it has been initiated by a Mobile Station (MS) of the communication network. In contrast, a co-called mobile terminated (MT) call is a connection which has been set up to a MS (Mobile station) or user terminal of a communication network, but which was initiated by the communication network.
In the case of a MO connection using the configuration shown in FIG. 1, a RAN node can connect to a secondary MSC in the case that the primary MSC is not available. However, for a MT connection the situation is wholly different. In this case, the MS is not reachable if it is attached to a faulty MSC. In such a situation the call to the MS will fail. Moreover as the address of the faulty MSC remains in the home location register (HLR) of the network, subsequent MT calls will also fail. There is currently no solution to this problem. All the MT calls will fail until the subscriber makes a MO operation or a location update.